Grains such as corn have long been used as a source of starch. One of the classic methods of separating the starch and then using it in other industrial processes is the wet-milling process. This method is a highly specific and integrated system developed to separate the major components of a grain kernel as completely as possible (see Stanley A. Watson, Starch: Chemistry & Technology, Vol. II, Industrial Aspects, Academic press, New York, 1967, pp 30-51). A final granular starch slurry coming out of the wet milling process can be used in a variety of industrial processes.
One of the most important processes is the conversion of starch to high fructose syrup. In practice, this conversion involves four major steps; namely liquefaction of granular starch slurry, saccharification of the liquified starch into dextrose, purification, and then isomerization of dextrose into fructose. The most common grain used in this process is corn in the production of high fructose corn syrup (see N. H. Aschengreen, et al; Starch Vol. 31, pp 64-66 (1979)). During the four step conversion to fructose, it is currently necessary that the granular starch slurry is varied widely in pH. The pH of the slurry coming out of the commercial wet-milling operation is about 4, then raised to a pH of from 6-6.4, and calcium is added along with enzyme. For saccharification of the starch the ph is lowered to 4.3-4.5 and for the final isomerization the pH is increased back to about 7.8. The result of these wide shifts in pH is a high ion exchange requirement to desalt the syrup during and after processing. Furthermore, high pH causes byproduct formation, sugar breakdown, color formation, and an overall decrease in product yield. These factors add millions of dollars annually to the cost of high fructose syrup production. The industrial isomerization process is currently very efficient due to current processing techniques and the short processing time. Accordingly, it would be useful if the liquefaction step could be carried out at lower pH's to obviate the need for a pH shift in commercial processes. It is possible to perform liquefaction at pH's less than 6 (see e.g. U.S. Pat. No. 4,376,824); however, the liquefaction is sometimes unexplainably incomplete and so has limited commercial utility. The antioxidant bisulfite has been used in the past as a component of starch slurry that is added during steeping operations to control pH and the nature of the fermentation. It is normally removed with the steep liquor.
Another source of starch is from the dry milling process. In the dry milling process the grain is ground and liquefaction can be carried on with or without separation of individual components of the kernel into grits, cornmeal or flour (see e.g. Corn: Chemistry & Technology by Alexander, K. , 1987, Chap. 11, Corn Dry Milling Productions and Applications). In addition to uses as in a variety of food and feed stuffs, he grain can also be used in the conversion of starch to alcohol. This process involves two steps to convert the milled grain or fractions therefrom into alcohol: liquefaction of the starch in the dry products after addition of water, and most generally a combined or simultaneous saccharification of the starch into glucose and fermentation of the glucose into alcohol.
The natural pH of the starch slurry of about 5 is adjusted upward with alkali or carbonate to a pH of 6.2-6.4 for liquefaction. After liquefaction the pH is adjusted downward by addition of acid or "backset" from previous fermentations. There are disadvantages to use of "backset" such as introduction of microbial contaminants into the fermentation.
The utility of the process of the invention is to reduce the need for pH adjustment on the up side and down side.